1. Field of the Invention
The present invention relates to a switching regulator which is frequently used as a power supply circuit, and particularly, to a start-up circuit of the switching regulator.
2. Description of the Related Art
As illustrated in FIG. 5, a switching regulator includes: an error amplifier circuit (error amplifier) 100 for comparing a divided voltage VFB obtained by dividing an output voltage Vout by a resistor R11 and a resistor R12 with a reference voltage VREF output from a reference voltage source 110; an amplifier 105 for converting a coil current flowing through a coil L into a voltage with the use of a resistor Rs; an adder 104 for outputting a signal SUM obtained by adding a voltage value which is obtained by converting the coil current into a voltage and a voltage value of a reference triangular wave signal; a PWM comparator (modulator) 101 for comparing a voltage value of an output signal Vc of the error amplifier circuit 100 with a voltage value of the signal SUM output from the adder 104 to generate a pulse signal of a duty cycle, which is in accordance with a comparison result; and a buffer circuit 103.
Switching control is performed on a p-channel type MOS transistor DH and an n-type MOS transistor DL which serve as a switching element in response to an output signal of the PWM comparator 101, and electrical energy stored in the coil L is adjusted (charging of the coil L is controlled), whereby the output voltage Vout is controlled to be constant at a predetermined set value.
In a current-mode switching regulator which is used for outputting a constant voltage, an output voltage of the error amplifier circuit 100 is slowly increased over a certain period of time in order to reduce an inrush current from a primary power source (battery) and an overshoot of the output voltage. This period of time is referred to as soft start time, and is generally set between several mS to 10 mS.
As described above, the output voltage of the error amplifier circuit 100 corresponds to an input signal which is input to one input terminal of the PWM comparator 101, which controls the switching transistor, via a soft start circuit 500. In addition, a signal obtained by converting the switching current of the coil L into a voltage by a predetermined amplification degree is input to the other input terminal of the PWM comparator 101.
As illustrated in FIG. 6, in the soft start circuit 500, a voltage Vin is input from a battery to thereby charge a capacitor 36, and a PNP bipolar transistor 37 becomes an off-state from an on-state to gradually increase the output voltage of the error amplifier. As a result, the switching current of the coil L increases in proportion to an increase of the output voltage, with the result that the output voltage Vout of the switching regulator is gradually increased.
In this manner, generally, the output voltage is increased from 0 V in a state of a light load for activating the switching regulator. In this case, in order to suppress the inrush current and reduce a burden placed on an input side of the power source such as a battery, a soft start circuit for gradually increasing the output voltage of the switching regulator is frequently used. The soft start circuit is realized by, in the PWM comparator, gradually increasing the voltage of the output signal of the error amplifier circuit, which is the voltage to be compared with the reference triangular wave signal, as described above. Specifically, as the soft start circuit 500, in many cases, the capacitor is charged with a constant current so that a voltage which gradually increases is generated to be output to the error amplifier circuit, whereby a soft start function is realized (for example, see JP 2007-159316 A).
However, in the above-mentioned conventional soft start circuit 500 used in the switching regulator, the output voltage Vout is changed with a constant slope within the predetermined soft start time, and hence it takes time before the output voltage Vout becomes constant.
On this occasion, when rise time in which the output voltage Vout increases to the predetermined voltage, that is, the soft start time is simply reduced, the inrush current from the primary power source and the overshoot of the output voltage may not be reduced.
Further, in a portable electronic device, the switching regulator has, as its function, a standby mode for stopping an operation of a stabilized power source in order to prolong life of a battery mounted thereon.
Unnecessary current is not consumed in high-speed switching from the standby mode to an operation mode in which the stabilized power source is operated, and hence there can be obtained a great power-saving effect in enabling the frequent operation in the standby mode.